Blender Material Color in Mitsuba (or any other) Shader
You can read the mat color from Blender Internal material like so:

# from material
mat = bpy.data.materials['Material_Name']
color = mat.diffuse_color

# or from object
ob = bpy.context.active_object
if len(ob.data.materials):
    # get 1st material slots material
    mat = ob.data.materials[0]
else:
    # no slots
    print("No Material")
    mat = None
if mat:
    color = mat.diffuse_color


To change LuxRenders material diffuse color:

mat.luxrender_material.luxrender_mat_matte.Kd_color = color


Mitsuba:

mat.mitsuba_material.mitsuba_bsdf_diffuse.reflectance_color = color


In Yafaray there is no need to change anything, it will use:

mat.diffuse_color